Modern infrared cameras operate over multiple bands in both the midwave and longwave radiation spectrums. In order to function successfully, the camera's detector surface must be enclosed in an IR transmissive window or domed enclosure. When used in service on an automotive or aerospace vehicle the enclosure must not only be IR transmissive but able to withstand considerable environmental exposure in the form of temperature extremes along with high speed wind, rain, ice, dust and dirt erosion. Such conditions will rapidly degrade soft, non-durable transmission windows through erosion and surface etching.
Beyond the direct needs of pure infrared systems, current advanced imaging systems look to combine detection capabilities in both the visible and IR wavelength ranges. This presents a requirement for moldable, durable window materials that are not only transparent in the IR, but also in the visible spectrum. There are relatively few pure materials with such broadband transmission and those that do exist are often ionic crystals or semiconductors typically resulting in brittle bulk material properties and significant aqueous solubility. These properties limit the material's potential for applications in which moldable, durable materials having the ability to withstand long term environmental exposure are desired.
For example, current state of the art IR transmissive windows such as germanium, BaF2, ZnS, ZnSe, CaF2, NaCl, KCl, Si, Saphire, MgO, MgF2, PbF, LiF, GaAs, fused silica, CdTe, AsS3, KBr, CsI, diamond, Thallium Bromoiodide (ThBrI), Thallium Bromochloride (ThBrCl), and Germanium Arsenic Selenide suffer from one or more of the following issues: opacity in the visual wavelengths, brittle crystalline behavior, difficulty of making windows that are of suitable size and also visually transparent, and/or being composed of hygroscopic salts. These properties often preclude their use in many environmentally challenging applications where exposure to heat, impact, and moisture is expected.
The vast majority of polymeric materials are highly IR absorptive in the wavelength ranges commonly employed in IR detectors and cameras. This is due to the interaction of common bond structures with IR wavelengths including esters, ketones, ethers, carbon-halogen bonds, and aromatic species. Thus, for fabricating IR transmissive materials, most commercially available polymer compounds will not work, with the exception of unsaturated hydrocarbon species, such as, for example, poly(ethylene). However, these hydrocarbon species typically suffer from lack of transparency due to crystallinity and low glass transition temperature properties. Thus, the use of polymeric materials for visual and IR transparent panels is limited by the tendency of the majority of commercially available polymeric materials (e.g., polycarbonate, polystyrene, Teflon, polyethylene, and polypropylene) to display one or more of the following shortcomings: broadband IR absorbance, visual opacity, and relatively low softening temperatures.
One IR transparent polymeric material is POLYIR® made by Fresnel Technologies. POLYIR is a collection of flexible plastic materials that display good transparency in multiple IR bands. However, POLYIR materials show significant visual haze or opacity, low maximum service temperatures and limited tolerance to sunlight and other environmental factors.
Thus, there is a need in the art for durable, rigid, visually transparent polymers that also demonstrate reduced absorption in both mid- and long-wave IR bands, and processes for making such compounds.